I am not, indeed, sure whether it is not true to say that the Milton who once seemed not unlike a seventeenth-century Shelley had not become, out of an experience ever more bitter in each year, more alien [sic] to the founder of that Jesuit sect which nothing could induce him to tolerate.

They’re completely automated, so no human is actually looking at the page and cutting and pasting. They are created to attract random web traffic in the hope of making money via advertising, or alternatively to set up a web of links (a "link farm") that all point to another site, thereby elevating that site’s ranking in Google and other search engines. It’s best to delete their automated comments — "comment spam" — either manually or through a blacklist (depending on your software). If their site displays Google ads you can report it to Google as a spam site and that will eventually get their advertising account deleted. As for copying all of your own content, there’s not a lot you can do unless you want to file copyright claims, and finding an actual person behind the scraper can be hard. But fortunately these sites generally last only a few months (after which others pop up to take their place).

Every ecosystem has not only producers but also parasites (blog scrapers, football coaches, etc.).